The present invention relates to a process for preparing alkali metal methoxides from aqueous alkali metal hydroxide, which may have been admixed with methanol, and methanol.
The preparation of alkoxides from alkali metal hydroxides and alcohols is known per se.
DE-C 968 903 has already described the continuous treatment of high-concentration aqueous alkali metal hydroxide solutions with a countercurrent of alcohol in a packed column, with the aqueous alkali metal hydroxide being able to be fed in at various places on the column in order to prevent blockage of the column by deposited alkali metal hydroxide. The alkoxide is formed by reaction of the alkali metal hydroxide located on the packing with the alcohol stream additionally passed through the column from the top downward.
However, low-boiling alcohols (methanol) can be used in this process only when a suitable auxiliary liquid is used for removing the water. This process also has the disadvantage that the deposits of solid alkali metal hydroxide which settle on the packing of the column can lead to blockage of the column. The procedure described in DE-C 968 903 does mention removal of the alcohol by azeotropic distillation using an entrainer, but in the procedure indicated the alcohol of the alcohol/water azeotrope which has been distilled off constantly has to be replaced, requiring a complicated recovery step.
SU-A 165 691 discloses a process for preparing sodium methoxide, which starts from a methanolic sodium hydroxide solution which is conveyed from a stirred vessel into the upper part of a continuously operated packed column in countercurrent to gaseous methanol.
The process requires a high energy input to achieve the appropriate product purity. It has the further disadvantage that solid sodium hydroxide is employed.
EP-A 0 299 577 describes a process for preparing alkoxides in which the water of reaction is removed by pervaporation. It is known that complete dewatering of the alcohol cannot be effected in this way.
DE-C 34 13 212 and DE-C 37 01 268 describe processes for preparing potassium tert-butoxide from aqueous potassium hydroxide and an excess of tert-butanol in a distillation column using an entrainer which, in contrast to the auxiliary liquid described in DE-C 968 903, does not form an azeotrope with water.
Furthermore, RO-A 60485 describes the isolation of sodium methoxide from anhydrous technical-grade methanol and technical-grade sodium hydroxide in a three-stage process with the aid of a hydrocarbon, in which process the water of reaction formed is removed by azeotropic distillation. The anhydrous hydrocarbon is recovered by fractional distillation with addition of Na2SO4. The sodium methoxide is obtained as a suspension in the hydrocarbon from which it can be filtered off.
These processes require additional separation operations and also leave extraneous materials in the alkali metal alkoxide solution. The latter interfere in further processing.
It has been found experimentally that the theoretical number of theoretical plates alone is not a sufficient criterion for an effective process for preparing substantially water-free alkoxide solutions. Thus, known processes for preparing alkoxides give only unsatisfactory qualities in which the water content is from about 0.5 to 2% based on the alkoxide, even in columns which have a high number of from 35 to 40 theoretical plates. These water contents are above those of commercial grades having a water content of about 0.1%.
It is an object of the present invention to provide a process for preparing alkoxide solutions which makes it possible to obtain a substantially water-free end product with justifiable energy input and in which no deposits are formed on the packing when a packed reaction column is used.
We have found that this object is achieved by the known process for preparing alkali metal methoxides from aqueous alkali metal hydroxide, which may have been admixed with methanol, and methanol in a reaction column having at least 5, preferably from 15 to 30, theoretical plates between the feed point for the aqueous alkali metal hydroxide and the feed point for the methanol, where in the case of a reaction column configured as a bubble cap tray, valve tray or sieve tray column, the trays are selected so that not more than 5%, preferably xe2x89xa61%, of the liquid rains through the respective trays.
For this embodiment of the process of the present invention, suitable columns are essentially bubble cap tray, valve tray and sieve tray columns. Specifically in the case of valve trays and sieve trays, the trays should be configured so that the raining-through of the liquid is minimized. A person skilled in the art will be familiar with the constructional measures required for this. Particularly tightly closing valve types are selected and thus, in particular, the vapor velocity into the tray openings is increased to double the value which is customarily set. This is achieved by a reduction in the number of valves. In the case of sieve trays, it is particularly useful to reduce the diameter of the openings in the tray and to maintain or even increase the number of openings.
In a variant of the present invention, the reaction column is provided with random packing elements or ordered packing, with ordered packing being preferred over random packing elements with a view to uniform distribution of the liquid. In this embodiment of the invention, the average ratio of liquid flow to vapor flow must not be exceeded by more than 15%, preferably not by more than 3%, in all subregions of the column cross section which correspond to more than 2% of the total column cross section. This low amount of liquid to be maintained according to the present invention obviously makes it possible for the capillary effect on the wire meshes to prevent local peak values of the liquid trickle density.
Suitable methods of achieving this are known from EP A 0 684 060. The desired characteristics of the liquid distribution can be achieved when using columns with random or ordered packing by the liquid trickle density in the marginal region of the column cross section next to the column wall, which region corresponds to about 2-5% of the total column cross section, being reduced by up to 100%, preferably by from 5 to 15%, compared to the remaining cross section. This can be achieved in a simple manner by targeted distribution of the drip points of the liquid distributors or their holes.
In this mode of operation, it is advantageous for the internal wall of the reaction column to be at a temperature which is from 3 to 10xc2x0 C. above the temperature of the reaction column.
Owing to the very unfavorable position of the chemical equilibrium, breakthrough of even very small amounts of still water-containing liquid which have not come into contact with a sufficiently large amount of vapor obviously has to be prevented with certainty. Local breakthrough of water-containing liquid can obviously have a serious adverse effect on the quality of the product due to backreaction.
The combination of the process known per se for preparing alkali metal methoxides with the features of the invention which ensure that passage at the margins or stream formation or raining-through of liquid occurs at no point of the reaction column cross section results, in a surprising and synergistic fashion, in a purity of the alkali metal methoxide solution which has hitherto only been able to be achieved in the amalgam process or by the use of alkali metals.
It may be stated, and is advantageous for industrial implementation, that there are no additional constructional requirements in respect of the uniformity of the gas flow over the cross-sectional area of the column. It is not disadvantageous if the ratio of gas flow to liquid flow locally assumes higher values than the average value. Rather, increased gas flows locally improve water removal.
When carrying out the process of the present invention, no blockage occurs in a reaction column provided with random or ordered packing, and the methanol used can be recycled to the process without complicated work-up.
The process of the present invention can be carried out either batchwise or continuously.
In batchwise operation, the alcohol and the water are vaporized to the point at which the desired alkoxide concentration is present in the reaction mixture.
In the continuous process, an aqueous alkali metal hydroxide stream, which may have been admixed with methanol, is fed in at the top of the reaction column. The reaction column is operated as a pure stripping column. Methanol is fed in vapor form into the lower region of the column. In-specification alkoxide is obtained via the bottom offtake. The water-containing methanol stream leaving the top of the column is worked up in the rectification column. At the top of this column, the fresh methanol required is introduced. After partial condensation, a gaseous methanol stream having a very low water content is obtained at the top of the column and is recirculated to the lower region of the reaction column. The water is removed from the system via the bottoms from the rectification column.
The aqueous alkali metal hydroxide used in the process of the present invention is obtained by an electrochemical or a membrane process advantageously by a membrane process. In this way, it is possible to produce alkoxides which are completely free of mercury. In terms of the energy consumption, it is also advantageous for the aqueous alkali metal hydroxide to have a concentration of at least 30% preferably concentrated to the solubility limit and for it to be heated by means of a heat exchanger to close to the boiling point at the pressure prevailing in the reaction column prior to entering the reaction column.
The methanol is used in such an amount that it simultaneously serves as solvent for the alkali metal methoxide obtained. This amount should be selected so that the desired concentration of alkali metal methoxide, preferably 30%, is present in the bottoms from the reaction column.
The abovementioned condition for the amount of methanol to be used is met when the amount is from 10 to 50 times, preferably from 35 to 40 times, the mass of water which is introduced with the aqueous sodium hydroxide, or from 10 to 50 times, preferably from 30 to 40 times, the mass of water introduced with the aqueous potassium hydroxide.
In the process of the present invention, the reaction column is operated without reflux.
In the rectification column, a reflux ratio of at least 0.5, preferably from 0.8 to 1.4, is advisable when the vapor taken off at the top of the reaction column is fed, preferably in gaseous form, into the rectification column and the methanol obtained at the top of this, which has a water content of from 20 to 100 ppm, is recirculated, preferably in gaseous form, through a partial condenser and is subsequently fed via a vapor compressor to the lower end of the reaction column.
In one variant of the process of the present invention, the reflux ratio of the rectification column is at least 0.6, preferably from 0.8 to 1.4.
In this case, the vapor taken off at the top of the reaction column is passed through a vapor compressor upstream of the rectification column and the methanol then obtained in the rectification column, having a water content of from 20 to 100 ppm, is fed, preferably in gaseous form, via a partial condenser to the lower end of the reaction column.
The dimensions of the reaction column can be reduced in the lower region of the column if part of the methanol is introduced in gaseous form at the upper end or in the region of the upper end of the reaction column.
In this mode of operation according to the present invention, only part of the methanol stream, namely from 10 to 70%, preferably from 30 to 50%, is fed in at the lower end of the reaction column and the remaining part is introduced, either as a single stream or divided into a plurality of substreams, in gaseous form from 1 to 10, preferably from 1 to 3 theoretical plates below the feed point for the aqueous alkali metal hydroxide.
The dimensions of the reaction column can be reduced in the lower region of the column by the optional installation of one or more intermediate vaporizers in the upper region of the reaction column. In this configuration, it is also possible to introduce substreams of the methanol in liquid form into the upper region of the reaction column.
It is advantageous to operate the reaction column at a pressure of from 0.5 to 40 bar, preferably from 1 to 15 bar, particularly preferably from 3 to 10 bar, since smaller heat inputs and smaller amounts of methanol can be achieved at higher pressure.
The pressure in the rectification column can be chosen freely within a wide range. It is advisable to work at from about 1 to 4 bar. In the case of vapor compression, the pressure change between the two columns is advantageously chosen so that vapor compression for the methanol/water mixture or alternatively the methanol stream is readily possible.
The methanol necessary for the reaction and for dilution of the alkali metal methoxide solution is introduced at temperatures up to the boiling point, preferably at room temperature, at the top of the rectification column.
In a further embodiment of the invention, which is advantageous in respect of capital cost, the reaction column and rectification column are accommodated within a single outer column wall in which the lower region of the column is divided by a vertical dividing wall installed between the column walls so as to prevent cross-mixing of liquid and vapor streams in this subregion of the column, both parts of this longitudinally divided region have their own vaporizer via which the alkoxide solution or the water is taken off, the methanol obtained at the top of the column at a reflux ratio of at least 0.5, preferably from 0.8 to 1.4, which has a water content of from 20 to 100 ppm, is passed through a partial condenser and vapor compressor and then introduced at the lower end of the column in the subregion from which the alkoxide solution is taken off.
In this mode of operation, it is found to be useful to operate the column at a pressure of from 0.5 to 10 bar, preferably from 1 to 8 bar, particularly preferably from 3 to 5 bar.